Schizophrenia is a serious brain disorder that affects approximately 1% of the human population. The cause of this complex and devastating disease remains elusive, although genetic, nutritional, environmental, and developmental factors have been considered. A combination of clinical, neuroimaging, and postmortem studies have implicated the dorsal prefrontal cortex (PFC) as a prominent site of dysfunction in schizophrenia.
Schizophrenia is typically characterized as a disorder of thinking and cognition, as contrasted to other disorders of mental faculties, such as mood, social behavior, and those affecting learning, memory, and intelligence. Schizophrenia is characterized by psychotic episodes during which an individual may lose the ability to test reality or may have hallucinations, delusions, incoherent thinking, and even disordered memory. There are varying forms of schizophrenia differing in severity, from a schizotypal disorder to a catatonic state. A review of schizophrenia can be found in Principles of Neural Science, 3rd ed., 1991, Kandel, Schwartz, and Jessel (Eds.), Connecticut: Appleton & Lange, pp. 853–868; of which Chapter 55 is incorporated herein by reference.
Diseases of organ systems, such as those of the heart, lung, and kidney, are usually confirmed by tissue pathology. A demonstrable pathology includes identifying and defining a structural abnormality in the organ, along with an associated alteration in organ function. This type of diagnosis is also utilized in certain neurological diseases. However, there are few psychiatric disorders in which clinical manifestations and symptoms can be correlated with a demonstrable pathology. The majority of mental illnesses are evaluated by observing changes in key behaviors such as thinking, mood, or social behavior. These alterations are difficult to ascertain and nearly impossible to quantify. However, progress is being made in diagnosing mental illness and in determining the neuropathology of mental illnesses.
The Diagnostic and Statistical Manual of Mental Disorders, Third Edition (DSM-III-R) and the updated DSM-IV, published by the American Psychiatric Association, represent the progress made in providing a basis for objective and rigorous descriptive criteria for categories of psychiatric disorders. While the DSM-III-R is very thorough and detailed, it is also quite lengthy. Thus, the process of reviewing the categories and applying them to data from a patient is also very time-consuming and arduous. In addition, there is no mechanism by which a patient can be diagnosed either as having or being susceptible to schizophrenia prior to the expression of symptoms. Thus, there is a longstanding need for an easy and definitive method for diagnosing schizophrenia. A diagnostic tool that can be applied prior to the expression of symptoms would also have great utility, providing a basis for the development of therapeutic interventions.
There is strong evidence for a genetic linkage of schizophrenia. Historically, there have been a number of studies on monozygotic twins of schizophrenics that indicated that 30–50% of the twins also had schizophrenia. The fact that this number is not 100% indicates that there are other factors involved in this disease process that may protect some of these individuals from the disease. It is apparent from a number of studies that the patterns of inheritance in most forms of schizophrenia are more complex than the classical dominant or recessive Mendelian inheritance. Recently, locus 1q21-22, a chromosome region containing several hundred genes, has been strongly linked to schizophrenia as shown by Brzustowicz et al., Science 288, 678–82, 2000, which is hereby incorporated by reference.
Until the 1950's there were no specific, effective treatments for schizophrenia. Antipsychotic drugs were identified in the 1950's, and these drugs were found to produce a dramatic improvement in the psychotic phase of the illness. Reserpine was the first of these drugs to be used and was followed by typical antipsychotic drugs including phenothiazines, the butyrophenones, and the thioxanthenes. A new group of therapeutic drugs, typified by clozapine, has been developed and were referred to as “a typical” antipsychotics. Haloperidol has been employed extensively in the treatment of schizophrenia and is one of the currently preferred options for treatment. When these drugs are taken over the course of at least several weeks, they mitigate or eliminate delusions, hallucinations, and some types of disordered thinking. Maintenance of a patient on these drugs reduces the rate of relapse. Since there is no way of determining if an individual is susceptible to schizophrenia, it is currently unknown if these antipsychotic compounds are useful in the prophylactic treatment of schizophrenia.
Signal transduction is the general process by which cells respond to extracellular signals (e.g. neurotransmitters) through a cascade of biochemical reactions. The first step in this process is the binding of a signaling molecule to a cell membrane receptor that typically leads to the inhibition or activation of an intracellular enzyme. This type of process regulates many cell functions including cell proliferation, differentiation, and gene transcription.
One important mechanism by which signal transduction occurs is through G-proteins. Receptors on the cell surface are coupled intracellularly to a G-protein that becomes activated, when the receptor is occupied by an agonist, by binding to the molecule GTP. Activated G-proteins may influence a large number of cellular processes including voltage-activated calcium channels, adenylate cyclase, and phospholipase C. The G-protein itself is a critical regulator of the pathway by virtue of the fact that GTPase activity in the G-protein eventually hydrolyzes the bound GTP to GDP, restoring the protein to its inactive state. Thus, the G-protein contains a built-in deactivation mechanism for the signaling process.
Recently, an additional regulatory mechanism has been discovered for G-protein signaling that involves a family of mammalian gene products termed regulators of G-protein signaling, or RGS (Druey et al., 1996, Nature 379: 742–746 which is hereby incorporated by reference). RGS molecules play a crucial modulatory role in the G-protein signaling pathway. RGS proteins bind to the GTP-bound Gα subunits with a variable Gα specificity as a substrate. RGS molecules shorten the GTP binding of the activated Gα subunits by acting as GTPase activating proteins (GAPs), accelerating GTP hydrolysis by up to one hundred fold. By the virtue of this GAP action and by making available the GDP-bound Gα to re-attach to βγ dimers, RGS proteins shorten the duration of the intracellular signaling. RGS proteins are expressed in nearly every cell; however, they show a tissue-specific expression across the body and cell type-specific expression in the brain. For example, RGS4 is strongly expressed in the central nervous system, moderately expressed in the heart, and slightly expressed in skeletal muscle (Nomoto et al., 1997, Biochem. Biophys. Res. Commun. 241(2):281–287 which is herein incorporated by reference).
Several members of the G-protein signaling pathways, most located downstream of RGS4 modulation, have been implicated in schizophrenia. Gil, Gq and Golf messenger RNA (mRNA) and protein levels all have been reported to be altered in various brain regions of the schizophrenic subjects. Furthermore, changes in expression of adenylate cyclase, phospholipase C, and protein kinases, as well as DARPP (dopamine- and cAMP-regulated phosphoprotein) phosphorylation changes are expected to be influenced by RGS regulation of Gα signaling. In addition, RGS modulation changes are expected to have significant effects on the signal transduction effected by neurotransmitters including dopamine, serotonin, GABA, glutamate, and norepinephrine.
An additional genetic marker of schizophrenia has been identified by Meloni et al. (U.S. Pat. No. 6,210,879). These investigators found that an allele of the microsatellite HUNTH01 in the tyrosine hydroxylase gene correlated with the expression of schizophrenia. However, the allele only appears to be present in sporadic schizophrenias.
There has been a long-standing need for a definitive and easy method for diagnosing schizophrenia as well as for an effective treatment with minimal side effects. Further, a need has been recognized in connection with being able to detect schizophrenia prior to the expression of noticeable symptoms.
A need has been recognized in connection with overcoming the various limitations to the current implementation of a method for diagnosing and assessing the susceptibility to schizophrenia are addressed through the use of the current invention.